In many contexts, especially in flue gas cleaning, electrostatic precipitators are the most suitable dust collectors. Their design is robust and they are highly reliable. Moreover, they are most efficient. Degrees of separation above 99.9% are not unusual. Since, when compared with fabric filters, their operating costs are low and the risk of damage and stoppage owing to functional disorders is considerably smaller, they are a natural choice in many cases. In an electrostatic precipitator, the polluted gas is conducted between electrodes connected to a high-voltage rectifier. Usually, this is a high-voltage transformer with thyristor control on the primary side and a rectifier bridge on the secondary side. This arrangement is connected to the ordinary AC mains and thus is supplied at a frequency which is 50 or 60 Hz.
The power control is effected by varying the firing angles of the thyristors. The smaller the firing angle, i.e. the longer conducting period, the more current supplied to the precipitator and the higher the voltage between the electrodes of the precipitator.
When separating dust of low or moderate resistivity, the degree of separation increases as the voltage between the electrode increases. The separation will thus be more effective at high voltage. The possible voltage is, however, not restricted by the construction of the high-voltage rectifier only, but also by the fact that at sufficiently high voltage, there will be flashover between the electrodes in the precipitator.
The optimal separation is therefore obtained when the voltage applied is just below the one causing flashover. Since the flashover limit may vary strongly according to varying operating conditions, a constant voltage is, unfortunately, not possible if one tries to obtain optimal separation, but instead one must frequently test the flashover limit by permitting flashover between the electrodes.
This is effected by slowly increasing the current until flashover occurs. Subsequently, the current is reduced in a predetermined manner and then again slowly increased until the next flashover. The procedure is repeated periodically. If the circumstances result in a highly varying flashover limit, more than 100 flashovers a minute may be acceptable. In more stable processes, 10 flashovers a minute may be involved. In certain processes, the best separation is however obtained at very high flashover frequencies although the operation is very stable. Up to now, this has not been explained in a satisfactory manner, but is verified by experience.
Examples of the technique of controlling are to be found in, inter alia, GB 1,402,149, FIG. 8 showing the fundamental reasoning. In case of flashover, the current is interrupted during a first time interval, and then the current is rapidly increased from zero, during a second time interval after which it is increased slowly when a given value, depending on the value before the flashover, has been achieved.
To ensure that the flashover does not lead to a permanent arc and, thus, sets the precipitator out of operation for a long time, the first time interval, during which the current is interrupted, must be at least a half-circle of the mains voltage. The current is usually interrupted during an entire cycle of the mains voltage, partly because otherwise the excitation of the transformer, when reconnected, yields a very high overload on the mains and increases the losses in the transformer windings.
This technique therefore implies that the precipitator is dead for 20 milliseconds up to 100 times a minute or even more frequently. Moreover, it will be appreciated that the separation is not fully effective also during the second time interval, when the precipitator is being recharged and the voltage between the electrodes is essentially below the value at which the flashover occurred. If the second time interval is estimated at about 100 milliseconds like in FIG. 8 of GB 1,402,149, the precipitator may, in extreme cases, be out of operation during almost as much as 10% of the total time. This is a strongly restricting factor at a high flashover frequency.
In conventional thyristor-controlled rectifiers, the current cannot be interrupted until the next zero point of the mains voltage. This means that the precipitator can function as a short-circuit load for a considerable time, between the flashover and the next zero point of the mains voltage. If the flashover occurs early during the half-circle, this state can prevail for almost 10 milliseconds.
To reduce the negative consequences of the wish of having a high flashover frequency, it is possible to operate with a higher frequency of the voltage, and, thus, via a converter avoid the dependence on the mains voltage. This has been suggested in e.g. DE 3,522,568, in which a voltage having a frequency of 2 kHz or more is generated in a converter, and in WO 88/00159 in which an embodiment states 50 kHz, but frequencies up to 200 kHz are mentioned.
By these methods, the time during which the current must be interrupted is reduced. It has proved sufficient to have an interruption of the current supply corresponding to the length of period also for these high frequencies. Instead of an interruption of 20 milliseconds, an interruption which is essentially shorter than 1 millisecond may thus be sufficient.
By these methods, also the loss of energy in the actual flashover is reduced. When the frequency is increased to e.g. 2 kHz, the current can be effectively interrupted as soon as after 0.5 millisecond or even earlier, at 50 kHz as soon as after 0.02 millisecond. This may not have any decisive influence on the total losses of energy, but the stress to which electric components and some mechanical components are subjected will be reduced.